To many, the notion of asteroids wiping out humanity may seem limited to the realms of Hollywood fantasy, but the reality is that many scientists believe there is a credible threat.

Recent years have seen a vociferous international group of asteroid worriers - made up of disparate alliances of astronomers, scientists and politicians - slowly shed their "doom-monger" image and begin to be taken more seriously.

Today's opening of the Near Earth Object Information Centre, in Leicester, is an indication of this. The centre, a first for the UK and backed by £300,000 of government money, will have a brief to study asteroids among other nearby heavenly bodies.

Relatively regular reports of "close shaves" have had something of a galvanising effect. In January, an asteroid came within 390,000 miles of earth, which is pretty close in intergalactic terms: if it had arrived four hours earlier in its orbit it would have scored a direct hit. Another asteroid, which could have wiped out an entire city, missed the earth by only 480,000 miles in 2000.

Right now, there is one "big one" out there and on course towards us that scientists know about. It emerged in April that there is a billion-tonne asteroid - measuring more than half a mile in diameter - in the vicinity with a one in 300 chance of hitting the Earth.

There is no immediate panic - there are 878 years before asteroid 1950DA arrives - but if it hit, it would leave a 14-mile wide crater and change the global climate.

Humanity would probably escape - but not unscathed. A bigger asteroid - more than six miles across - is believed to have wiped out the dinosaurs 65m years ago when it plunged into the sea off Mexico.

Nasa says it would be possible to destroy an approaching asteroid, or knock it off course, but it would need early warning: the key concern of those anxious about spending money on extending the Earth's "spaceguard" resources.

Astronomers claim that there are some 2,000 sizeable asteroids in orbits that could hit the Earth, but they only know where around 200 of them are. If one sneaked past the current array of telescopes looking for them, the worst case scenario is that we would have only 20 seconds warning before "deep impact".

Those most worried are campaigning for a global safeguard system where more telescopes are built to look out for asteroids and a globally linked system set up to give an infallible early warning. At the moment, there are blind spots.

The US has three telescopes scanning the northern hemisphere but more are needed in the southern hemisphere, according to campaigners. However, telescopes powerful enough to be useful cost tens of millions of pounds, and while governments may find funds for study centres, detecting an asteroid-Armageddon is not yet seen as much of a vote winner.

Asteroid facts

· Some scientists claim that humans have a one in 5,000 chance of being wiped out by an asteroid in the next century.

· Asteroids are made of rock and iron.

· There are more than 700,000 asteroids big enough to destroy us that we know about. Asteroids larger than 50 metres across strike the earth on average once a century. The last was in Siberia in 1908 - if one of the same size hit London, it would destroy everything within the M25.

· A 10km asteroid is believed to have killed the dinosaurs 65m years ago.

· An asteroid over one kilometre in size would release energy equivalent to 10m times the power of the Hiroshima atomic bomb if it hit the earth.

· Every week the earth collides with thousands of tonnes of dust and stones hurtling through space at up to 20 miles a second. Every few years, large boulders hit the outer atmosphere and burn up.